This image provided by the journal Science shows the skeleton, with wing and tail feather impressions, of the tenth specimen of the first known bird, Archaeopteryx, in ventral view. The new specimen provides important details on the feet and skull of these birds and strengthens the widely but not universally accepted argument that modern birds arose from theropod dinosaurs. (AP Photo/Science)
Ran on: 12-02-2005
This new archaeopteryx specimen shows skeleton and feather impressions, and it provides new details about the animal's feet. This image provided by the journal Science shows the skeleton, with wing and tail feather impressions, of the tenth specimen of the first known bird, Archaeopteryx, in ventral view. The new specimen provides important details on the feet and skull of these birds and strengthens the widely but not universally accepted argument that modern birds arose from theropod dinosaurs. (AP Photo/Science)

Photo: Science, AP

This image provided by the journal Science shows the skeleton, with...

The public birthday bashes and learned lectures are pretty much over, but 2009 remains the 200th anniversary of Charles Darwin's birth and the 150th anniversary of the publication of his iconic magnum opus.

"On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life," known more simply as "Origin of Species," was Darwin's masterwork, and in it he describes in extraordinary detail how species of plants and animals descend from earlier species by adapting to the pressures that changing environments impose on them. Species unable to adapt die off while new forms carry on the races.

"Origin's" first edition of 1,200 copies was an instant sellout and provoked historic debates between science and religion. It still does today, although Darwinian evolution has been confirmed by science repeatedly.

Skeptics protest that, in tracing life's evolutionary past, few if any examples exist of "transitional forms" of fossils - that is, fossils showing a wing as one step toward becoming an arm in the far distant future, or a fin on the way to becoming a leg.

In fact, transitional forms have been found again and again in millions of years of the fossil record.

The British scientific journal Nature recently published a list of what it called "evolutionary gems" - recent discoveries that have demonstrated classic examples of transitions from one ancestral form to entirely different ones that evolved much, much later.

Some fossil examples are obscure and barely visible, but many are clear both to researchers and lay enthusiasts. The journal's editors listed the fossils of a land-living, hippo-like creature, for example, that evolved into to the primitive version of a whale. And a primitive fossil fish showed clear transitional forms of what would much, much later become legs for walking on land.

Apart from the Nature list, other recent cases of evolutionary transitions have been widely reported. Here are some of them:

Ardipithecus to Homo sapiens

Tim D. White, a UC Berkeley paleontologist, sees evidence of evolution in the Ethiopian hominid fossils that preceded our own humankind by millions of years. He has dug up the bones of Australopithecus and Ardipithecus hominids - varied species that walked the Earth 1 million to 6 million years ago. Evidence shows that Australopithecus - among them the famed Lucy - was a dead-end tribe, but Ardipithecus clearly gave rise to the "Homo" lineage that ran in transitions from Homo habilis to Homo erectus and finally, White says, to Homo sapiens - us.

Dinosaurs to modern birds

A story of evolution with more ancient roots links the dinosaurs of old to today's birds. It started with the iconic winged dinosaur, called Archaeopteryx, of 150 million years ago, evolving as the world's first-known flying proto-bird. "Birds, including Archaeopteryx, are classified as feathered dinosaurs," says Kevin Padian, a UC Berkeley paleontologist who specializes in their evolution. "We definitely have seen a progression of evolution in the feather types of these dinosaurs, beginning with simple hairlike structures, to branched and downy forms and then to feathers with vanes, barbs and a central stalk."

Tale of the warblers

Then there's the curious story of the Himalayan warblers.

Eight years ago, a Chronicle story told of three California scientists, led by Darren Irwin of UC San Diego, who had tracked a species of birds called "greenish warblers" around the vast Himalayan plateau from the area where the species originated some 10,000 years ago. The scientists discovered that members of that species had, at some point, split off and migrated northward into new habitats of the plateau. Much later, descendants of the two groups met again north of the Himalaya in Siberia, where they live now.

It was there that Irwin discovered that the eastern and western warbler tribes now differed subtly in the colors of their plumage and sharply in their songs. And they do not interbreed - a sure sign that the original warbler species had evolved into separate and distinct species, Irwin concluded.

He was right. When Irwin and his colleagues studied warblers around the Himalayan plateau, they found in almost every case that the birds had altered gradually in color and song in each separate new habitat. They differed subtly, but were not yet distinct species.

Later, in the lab, Irwin and his team found that the genes of those birds had undergone small but distinct molecular changes along their divergent habitat paths - and that the DNA differences were greatest between the eastern Siberian warblers and the western ones.

This was confirming evidence of evolution at the genetic as well as the physical level, and Irwin and his colleagues have published several reports on their research in Nature and many other journals.